This invention relates to a method and apparatus for controlling the speed at which tape is transported in a video tape recording and/or reproducing machine, more particularly, to a method and apparatus for generating a synthetic control track signal from vertical synchronizing information included in a video signal reproduced from the tape for use in servo controlling the transport of the tape.
Magnetic video tape recording and/or reproducing machines generally operate by transporting magnetic recording tape in transducing relation to one or more rotating transducers or heads which scan the tape at a very high rate relative to the speed of transport of the tape. The heads record a plurality of generally parallel tracks on the tape which contain video information, and the same heads or other heads reproduce the recorded video information during playback by scanning the recorded tracks.
During recording, a control track head records control track signal information typically in a track extending longitudinally along one side of the tape, simultaneously as video information is recorded by a rotating video head scanning along the tape at locations spaced from the longitudinally extending control track. The control track signal information typically is a television frame rate square wave. For many helical wrap video recorders which record a field of television or video information along a single track during each revolution of the video head, the frame rate control track signal changes state once for each field of recorded video information. The control track signal information also often includes periodic color framing pulses for identifying specific fields in a multifield sequence forming the color television signal.
The longitudinal transport speed of the tape must be accurately controlled during playback. To achieve such control, many video tape recorders employ a motor driven prime mover, such as a capstan controlled by a capstan servo mechanism in accordance with a tachometer signal and with the timing relationship between a reference vertical sync signal and control track signal information reproduced from the tape during playback.
If for any reason a control track signal is improperly recorded relative to the recorded positions of the video tracks along the tape, the video head will be mispositioned relative to the recorded video track during playback operations, although the capstan servo will cause the tape to be transported at the proper playback speed. If the control track signal is not recorded on the tape, the capstan servo may not be able to move the tape at all, and even if it is able to do so, the tape transport will not be properly synchronized to the controlling reference. For example, control track information may be absent because of not being recorded on the tape during an insert edit or of, malfunctions and the like. Inaccurate control of the transport of tape also occurs if, the control track information is recorded with an improper inverted phase.
Recent developments in helical wrap video tape recording and reproducing machines have included what is commonly referred to as an automatic scan tracking system. In such machines, the reproducing head is mounted at the end of a movable element that is controllable to move the head transversely relative to the longitudinal direction of a recorded track so as to maintain the head on the track. This enables reproduction of a signal of superior quality, even when the tape is being transported at a speed other than normal reproducing speed, such as during operations for generating special motion effects of still frame reproducing, slow motion reproducing and fast motion reproducing. Such systems generate an error signal indicating whether the head is mistracking and the error signal is used in a head positioning servo for moving the movable element to return the head to the center of the recorded track. While the machine is reproducing recorded information, the error signal can be monitored to provide an indication of the tape speed deviation from normal reproduce speed as well as head to tape position deviation. Moreover, this error signal can be used to control the capstan servo to generally maintain proper tape speed and proper head to tape position. However, this error signal is not entirely reliable in providing control of the capstan servo because it does not rely on any well defined timing signals and is subject to mechanical and electrical variations which can affect the accuracy of the control of the capstan servo.
Accordingly, an object of this invention is to provide a new and improved method and apparatus for generating a synthetic control track signal for use in controlling the speed of tape transport in a video tape recording and/or reproducing machine whereby the adverse effects of improperly recorded or missing control track signals are obviated.
Another object is to provide a new and improved synthetic control track signal which is generated from vertical synchronizing signal information included in the video information reproduced from tape.
Yet another object is to provide a new and improved method and apparatus for generating a synthetic control track signal during video playback operations for use in synchronizing the transport of the tape to system reference while simultaneously recording a new control track signal on the tape.